Project Summary Invariant Natural Killer T (iNKT) cells are a type of immune cell that is numerally small but immunologically important to our health. Numerous studies in humans and mice have suggested that if you do not have a normal population of iNKT cells, you have an increased susceptibility to autoimmune diseases and cancers. In the mouse, two genes with 95% sequence homology are encoding for the CD1d molecules. Recognition of lipid antigens presented by CD1d molecules is absolutely required for the development and functions of iNKT cells. Early findings argued that the second CD1d-encoding gene, CD1D2, does not play any role in iNKT cells biology. However, in complete departure from the current dogma, our preliminary data show that the CD1D2 gene is expressed in certain mouse strains. Furthermore, the crystal structure of the CD1d2 protein and our preliminary functional data, show that the antigen-binding groove of CD1d2 is restricted in size compared to CD1d1 and does not accommodate presentation of antigens with long acyl-chains. Although the lipid portions of antigens are buried in the CD1d groove, several reports have shown that the composition of the fatty acids, including the length, the degree of insaturation and the presence of other modifications, strongly influence antigenic potency. The proposed studies aim at examining how CD1d1 molecules engineered to mimic antigen presentation of CD1d2 molecules affect the development of iNKT cells in vivo. These studies will open up substantial new possibilities for understanding iNKT cell development and effector programming so that it can be exploited for therapeutic usage to improve human health.